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The present invention relates generally to orthopaedic implants, and, more specifically, to an orthopaedic implant having a porous biocompatible metal surface layer, or pad.
It is known to provide an orthopaedic implant covered by a porous layer to promote ingrowth of bone within the openings of the porous layer, thus providing long-term fixation of the implant to the bone. Several methods of accomplishing such bonding are also known.
Diffusion bonding is the most widely known and accepted method of attaching a porous layer to an implant. Diffusion bonding of porous layer to a substrate having a curved surface, such as a portion of a hip stem, however, is not without difficulties. For example, diffusion bonding a porous material to a substrate can cause notches to be formed in the substrate, thereby decreasing the strength of the substrate. Orthopaedic implant manufacturers have successfully compensated for the effect diffusion bonding by either limiting the amount of porous layer attached to the substrate, or by adjusting the size and shape of the substrate to increase its strength.
Nevertheless, diffusion bonding a porous metal pad to an implant body is both time-consuming and expensive from a manufacturing standpoint. For example, the ramp-up and cool down time for a furnace necessary to conduct a diffusion bonding process may be as high as 14 hours per cycle. In some applications, such as diffusion bonding a porous metal pad to the interior bone-engaging surface of a femoral knee component, it may take a minimum of three cycles (42 hours) to complete the diffusion bonding operation.
In order to avoid some of these problems, the Applicant has developed an improved process for bonding a porous metal pad to the surface of an orthopaedic implant comprising diffusion bonding a fiber metal pad to a thin metal foil, then attaching the metal pad to an orthopaedic implant using a laser welding process. (For details of such an attachment process, reference is hereby made to U.S. Pat. No. 5,504,300 entitled xe2x80x9cOrthopaedic Implant and Making of Making Same;xe2x80x9d U.S. Pat. No. 5,672,284 entitled xe2x80x9cMethod of Making Orthopaedic Implant by Welding;xe2x80x9d U.S. Pat. No. 5,773,789 entitled xe2x80x9cMethod of Making Orthopaedic Implant Having a Metal Pad;xe2x80x9d and U.S. Pat. No. 6,049,054 entitled xe2x80x9cMethod of Making an Orthopaedic Implant Having a Porous Metal Pad,xe2x80x9d each of which is assigned to the assignee of the present invention and incorporated herein by reference. In general, each of these patents describes a method for attaching a porous surface to an orthopaedic implant, wherein a porous metal pad, such as a fiber metal pad, is diffusion bonded to a thin metal foil. The fiber metal pad is then configured to be received within a recess formed in an orthopaedic body. The edges of the thin metal foil extend to the exterior of the recess formed in the orthopaedic implant body. A laser welder is used thereafter to weld the thin metal foil to the orthopaedic implant body in various positions, thereby indirectly attaching the fiber metal pad. Laser welding, however, while in some ways, superior to diffusion bonding, or sintering, a porous metal pad directly to an implant body, does require expensive laser welding devices.
Due to the difficulties associated with known means of bonding a porous surface to the core of an orthopaedic implant, attention has been focused on the use of nonmetallic materials for constructing all or a portion of an orthopaedic implant and then bonding the porous metallic surface thereto. Materials such as plastics, composites, ceramics, and thermosets have been used in such devices. Such materials are used in the hope of better matching the flexural rigidity of bone and gaining greater transfer of forces through the implant to the natural bone in order to promote bone growth and stability. U.S. Pat. No. 5,236,457 to Devanathan; and U.S. Pat. No. 5,571,187 also to Devanathan, both assigned to the Applicant, disclose methods of bonding a porous metal and to a polymer layer.
In general, these patents describe a method for producing an orthopaedic implant by providing either a metal core surrounded by a polymer or a completely polymeric implant body; forming a porous surface layer insert; positioning both objects in a mold; and injecting a polymer there between in order to bond the porous surface to the implant body. Thereafter, amorphous polymers that are used to form the porous surface are removed from the implant using a solvent. Although this method produces a successful and safe product, it is desirable to produce a process that does not require extracting amorphous polymer from the porous outer surface, thereby alleviating the necessity of maintaining costly and potentially dangerous solvents at the manufacturing facility. A need exists, therefore, for a method of impregnating an interior surface of a porous metal pad with a polymer film, similar in chemistry to the polymer used in the bonding step. This improvement does not require the use of a solvent to remove amorphous polymer from the outer surface of the impregnated pad. A further need exists for bonding such an impregnated pad with an orthopaedic implant to form an orthopaedic implant having a porous metal surface.
The invention comprises a method for bonding a porous metal surface to an orthopaedic implant having a plastic body or a substantially solid core surrounded by a plastic body. In addition, the present invention provides a method for impregnating a porous metal pad with a polymer film, such as a polyaryletherketone, to facilitate later bonding of the porous metal surface to a polymer surface.
The method of impregnating a porous metal pad comprises providing a sheet of biocompatible metal, preferably, commercially pure titanium, a sheet of heat-resistant releasable plastic film, generally a polyimide film such as KAPTON(trademark); a sheet of semi-crystalline polymer film, such as polyetheretherkeone, polyaryletherketone or polyphenylenesulfide; a porous metal pad, a second sheet of heat-resistant releasable plastic film and a second sheet of biocompatible metal.
These sheets and the pad are arranged such that the biocompatible metal sheets from the outermost layers and such that the porous metal sheet abuts the semi-crystalline polymer film. The layered arrangement is subsequently positioned into a press for sufficient time and under sufficient conditions to allow the semi-crystalline polymer film to interpose itself within the porous metal, thus producing an impregnated pad.
The method of forming an orthopaedic implant having a porous metal surface comprises providing a porous metal pad of desired size and shape suitable for bonding to an orthopaedic implant, impregnating the porous metal pad, using the method described above, placing the impregnated porous metal pad into a molding device, then molding a polymer into the molding device such that the polymer forms the body of an orthopaedic implant and bonds to at least one surface of the porous metal layer.
In an alternative embodiment, a substantially solid core is placed into the molding device prior to molding a polymer. The core may comprise any biocompatible material including titanium, or a ceramic; however, the core preferably comprises a cobalt chromium molybdenum alloy. When the polymer is molded in the device, it surrounds the core and acts as an intermediary layer between a biocompatible metal core and the porous metal surface of an orthopaedic implant.
In this manner, the present invention provides a method of making an orthopaedic implant having a porous surface without the shortcomings of either laser welding or diffusion bonding. In addition, semicrystaline polymer reduces the processing time require to produce an implant.
These and other advantages of the present invention will be apparent to persons skilled in the art upon reading the present specification and the appended claims.